One common type of seismic survey is a “towed marine” seismic survey, in which a “spread” of one or more streamer cables (or just “streamers”) having seismic receivers disposed along their length are towed through water by a survey vessel. A source array is also towed through the water, either by the survey vessel or by a separate source tow vessel.
Various factors influence the depth at which the streamer is towed. A streamer towed at a shallow depth is more susceptible to environmental noise, so that the signal-to-noise ratio in acquired data is higher for data acquired at a deeper streamer. However, the deeper is the streamer, the greater is the attenuation at high frequencies.
One feature that influences the choice of streamer depth is the phenomenon known as the “ghost” effect. As is known, the “ghost effect” at a receiver disposed in a water column occurs as a result of interference between a seismic signal arriving at the receiver directly from reflection at a geological feature within the earth and a seismic signal from that geological feature that has travelled to the surface of the water column and has been reflected at the surface of the water column back to the receiver. The ghost effect causes variations in the amplitude of a recorded seismic signal, and causes ghost “notches” at which the recorded amplitude become zero at specific frequencies at which there is a destructive interference of the wavefields travelling in opposite directions at the receiver.
As reflection at the sea surface is negative (ie, there is a phase change of π upon reflection at the sea surface), for the case of normal incidence, i.e., vertically travelling waves that will be travelling at normal incidence to the sea surface, the ghost notch frequency is given by:fnotch=nc/2h  (1)where n is an integer such that n≦0, h is the streamer depth and c is the velocity of sound in the water column (c is sometimes referred to as the “water velocity”, but it should be noted that it does not denote the speed of movement of water in the water column).
FIG. 2(a) shows the amplitude plotted against frequency for receivers at two depths. A shallow streamer has the first-order (n=1) ghost notch at a high frequency, but there is considerable attenuation at low frequency owing to the zero-order ghost notch at 0 Hz (see the line “a” in FIG. 2(a)). As a result the shallow streamer has a poor signal-to-noise ratio in the low frequency part of the spectrum, and this can present problems as low frequency information is often of interest in seismic surveys. On the other hand a deep streamer has a strong low frequency response (there is less attenuation at low frequencies), but the first-order (n=1) ghost notch occurs at a relatively low frequency (see the line “b” in FIG. 2(a)). This also presents problems in seismic surveying, as the seismic bandwidth in a typical survey (that is, the frequency range of interest) extends up to 100 Hz and above.
In summary, the ghost effect influences the tow depth used for the streamer spread-towing a streamer at a shallow depth provides for good higher frequency acquisition (i.e., good signal-to-noise ratio at the higher frequencies), but at the expense of attenuation at lower frequencies, whereas towing the streamer at a deeper depth provides for better acquisition at lower frequencies (i.e., good signal-to-noise ratio at the lower frequencies) at the expense of attenuation of other frequencies within the seismic bandwidth.
It should be noted that the above description relates to ghost effects at the receiver, ie to receiver-side ghost effects. Ghost effects may also occur at the source, leading to “source ghost notches”—that is, to notches in the spectrum of energy emitted by the source.
Compensating for ghost effects has been the subject of geophysical research for many years.
One solution for compensating for the receiver-side ghost effect is referred to as over/under acquisition. In over/under acquisition, streamers are towed as vertically aligned pairs and seismic data acquired at the two streamers of a pair are combined to achieve the deghosting step (see for example, B. J. Posthumus, Deghosting Using a Twin Streamer Configuration, Geophysical Prospecting, 41, 267-286, 1993, the content of which is hereby incorporated by reference.) Another solution uses streamers at only one depth, with the streamers having receivers that record both pressure and particle velocity measurements. The pressure and velocity measurements from the streamers are combined to achieve a deghosting step (see for example, Andrew Long, Dave Mellors, Terry Allen, and Avon Mc Intyre, A Calibrated Dual-Sensor Streamer Investigation Of Deep Target Signal Resolution And Penetration On The NW Shelf Of Australia (CH 2.7) 78th SEG 2008, the content of which is hereby incorporated by reference).
Because of the streamer arrangement, the over/under method of receiver-side de-ghosting requires twice as many streamers to cover the same spread aperture as a traditional streamer system, with a corresponding decrease in acquisition efficiency. Moreover, a tow vessel is generally limited in the number of streamers that it can tow, so that towing streamers in an under/over arrangement requires either an increased streamer spacing or a reduced spread width compared to a single depth streamer array. The pressure plus velocity method of Long et al. (above) requires new hardware, and suffers from high levels of noise in velocity measurements at low frequencies, rendering such measurements un-useable below a cut-off frequency (so that, below this frequency, the method reduces to a deep tow pressure measurement).
A proposed new solution for the ghost effect is described in WO2008102134 (the entire content of which is hereby incorporated by reference for all purposes). WO2008102134 describes a method that is referred to as the “sparse under method”. In the sparse under method of WO2008102134, shallow towed streamers are used in combination with a smaller number of deeper towed streamers. This is in contrast to traditional over/under acquisition as described above, in which both over- and under-streamers are towed at greater depths than the shallow depth streamers of the sparse under method and the streamers are towed in pairs.
The method of WO2008102134 is illustrated schematically in FIG. 1. This is a vertical cross-section through a seismic survey, perpendicular to the streamer direction (that is, the streamers extend into the plane of the paper in FIG. 1). A plurality of “over streamers” 1 (also denoted as S1 . . . S6) are towed at a shallow depth, in the example of FIG. 1 at a depth of 7.5 m below the surface 3 of the water column. A plurality of “under streamers” 2 (also denoted as L1 and L2) are towed at a deeper depth, in the example of FIG. 1 at a depth of 15 m below the surface 3 of the water column. There are fewer under streamers 2 than over streamer 1. As a result, some of the over streamers 1 can be considered as being “paired” with a respective under streamer—in the example of FIG. 1, the over streamers S2 and S5 can be considered as being “paired” with under streamers L1 and L2 respectively. The other over streamers S1, S3, S4 and S6 can be considered as “unpaired”, as they are not paired with any of the under streamers. The over streamer and the under streamer forming a “pair” are preferably disposed in a common vertical plane (to within the usual tolerance within which streamers can be deployed in an actual marine survey).
The key difference between the method of WO2008102134 and the conventional method is that, in the sparse under method, the use of a shallower tow depth for the upper streamers provides for optimization of the mid- and upper-frequencies in the acquired seismic survey. In the sparse under method, a smaller number of deeper cables are positioned at a depth that optimizes the low frequencies only. Combining the two datasets, for example by merging low frequency data from the deep towed cables 2 with high frequency data from the shallow towed cables 1 provides broad-band data with good signal-to-noise ratio at both the high and low ends of the spectrum, while requiring fewer streamers than in a conventional under/over survey.
The effect of the method of WO2008102134 is illustrated with reference to FIG. 2(b). The left panel of FIG. 2(b) shows typical stacked seismic data acquired at a streamer towed at a depth of 5 m below the surface of a water column (an “over” streamer), and the right panel of FIG. 2(b) shows typical stacked seismic data acquired at a streamer towed at a depth of 18 m below the surface of a water column (an “under” streamer). FIG. 2(b) shows stacked data for frequencies below 5 Hz and it can be seen that, even at these very low frequencies, there is a very good useable signal-to-noise ratio from the streamer towed at a depth of 18 m. In contrast, the data acquired by the streamer at a depth of 5 m is of low quality at frequencies below 5 Hz, and this is because the prominent ghost notch at leads to a very poor signal-to-noise ratio at low frequencies. In the method of WO2008102134, the low frequency data acquired by the under streamer is combined with the high frequency data acquired by the over streamer. (It should be noted that the precise “cross-over frequency”, above which data from the over streamer are used and below which data from the under streamer are used, will depend on the actual signal-to-noise ratio for real data, and so cannot be accurately established from the ghost responses alone.)
With regard to the zero frequency notch, the zero frequency notch is present in all of the described solutions for compensating for the ghost effect, but all solutions provide for enhanced low frequency content/acquisition compared to standard shallow towed streamer spreads.